Touchscreens are used in a number of data processing devices. Examples of such data processing devices include touchscreen kiosks, point of sale systems, personal digital assistants, and tablet PCs. Various types of touchscreen technologies can be used to display information to a user and allow the user to interact or manipulate data displayed on the touchscreen. FIG. 1 is a block diagram of a touchscreen system according to the prior art. Touchscreen 100 includes display 102 with overlay 104 covering the screen of display 102. Overlay 104 can be implemented as one of several different types of overlays, including a pressure-sensitive overlay, an electrically-sensitive overlay, or photosensitive overlay.
A pressure-sensitive or resistive overlay is implemented with an electrically conductive and resistive metallic layer covering a glass panel. An electric field is disrupted when a user touches the screen. This disruption is used to determine the location on the display that was touched by the user.
An electrically-sensitive or capacitive overlay is implemented as a capacitive layer that stores an electrical charge. When a user touches the display, charge is transferred to the user's finger causing a reduction in the charge on the capacitive layer at the spot touched by the user. This reduction is measured by circuits located at each corner of display 102. The location where the user touched display 102 is determined using the relative differences in charge at each corner.
A photosensitive or optical imaging overlay is implemented with image sensors placed around two edges of display 102. Light sources are placed on the edges of display 102 that oppose the edges with the image sensors. When a user touches the display, the touch is captured by the image sensors as a shadow or dark spot and each pair of image sensors can then be triangulated to locate the spot on display 102 touched by the user.
Unfortunately, overlay 104 can reduce the visibility and clarity of an image displayed on display 102. And all three of these techniques must be touched with a finger or stylus only at the desired point of contact. If a user inadvertently rests his or her hand or wrist on the touchscreen, the touchscreen can incorrectly interpret this contact as an intentional point of contact. This erroneous interpretation by the touchscreen can result in mistakes during data entry or data processing. And finally, the size of overlay 104 increases as the size of display 102 increases. This increases the cost to construct and maintain a touchscreen system.